Thaw cod and chop into bite sized pieces. If using salmon, do the same and remove the skin.
Bring a large saucepan of salted water to a boil. Add potatoes and cook for about 20 minutes or until very tender. Drain well and place in a bowl. Mash with 3 tbsp butter until well combined. Season with salt and pepper and set aside.
While the potatoes boil, begin to heat 4 tbsp butter in a skillet over medium heat. Sautée the onion for 5 minutes or until soft. Add cheddar cheese, cream, Dijon mustard, and lemon juice. Stir frequently as it cooks, for about 3-5 minutes or until the cheese has melted. Remove from heat.
Add the cod to the skillet and stir until well combined.
Preheat oven to 400 degrees.
Transfer filling to a pie pan and spread the mashed potatoes over the top to create a crust.
Bake until sauce is bubbling and potato crust is golden, about 45 minutes.
Garnish with chives.
Serve hot and enjoy!